Due to the recent progress in the fields of genetic engineering and molecular biology, infectious or genetic diseases can be diagnosed at the DNA or RNA level. In particular, by virtue of the development of nucleic acid amplification methods, including polymerase chain reaction method (PCR method: Science, 230:1350-1354, 1985) and NASBA method (Nucleic Acid Sequence Based Amplification method: Nature, 350, 91-92, 1991; Japanese Patent No. 2648802, and Japanese Patent No. 2650159), detection of an infinitesimal amount of nucleic acid present in a biological sample, which has been difficult so far, has become possible, thereby dramatically facilitating genetic analyses.
For example, in the field of oncology, diagnosis of tumor metastasis to lymph nodes has tremendous significance. As an approach to diagnosis of tumor metastasis to lymph nodes, there is a method for detection of tumor marker protein such as cytokeratin (CK). Due to the recent development of genetic analysis technology, detection of mRNA expression of tumor marker protein has allowed effective diagnosis of tumor (The Hokkaido journal of medical science, vol. 66(2), pp. 135-141, 1991). RT-PCR has allowed detection of CK mRNA expression in excised tissue, and underdiagnosis of tumor metastasis can be prevented to some extent. These nucleic acid amplification methods have been put to practical use in the field of diagnosis of tumor and cancer (Manual of Clinical Laboratory Medicine, 31st ed., Kanehara & Co., Ltd, published on Sep. 20, 1998).
As another DNA amplification method, the LAMP method has been reported (Patent Document 1). The LAMP method is a gene amplification method using multiple primers including those forming a hairpin structure at the ends of the amplified product as the strand displacement reaction proceeds. First, in the primary reaction, a dumbbell-like structure with loops at both ends is synthesized from template DNA by using a pair of inner primers (FIP and RIP), a pair of outer primers (F3 and R3 primers) and strand-displacement DNA polymerase. This structure serves as the starting structure for the amplification cycle, and the elongation and synthesis reactions progress from the 3′ end of the structure using itself as a template. The amplified product is composed of a number of repeat structures, each unit of which comprises, within the strand, the complementary regions linked in the inverted direction which are derived from the nucleotide sequences of double-stranded nucleic acids corresponding to the amplified region between the primers. The LAMP method has features that heat denature of double-strands to single-strands is not required and that all the amplification reactions can proceed consecutively at a fixed temperature (Non-patent Documents 1 and 2). Likewise, when the template is RNA, the starting structure can be synthesized by adding reverse transcriptase to the composition of the reaction solution for DNA template, and the amplification can be conducted (RT-LAMP method). The LAMP method gives a sufficient amount of amplification product to be detected in about 30 minutes. Thus, this method may be applied to the diagnosis of tumor metastasis to lymph nodes for the purpose of prompt determination of the therapeutic strategy, because the time required for detection of nucleic acids is reduced. This method is also promising for application to intraoperative diagnosis, since it can give prompt results.
To quantify mRNA, mRNA of a housekeeping gene, whose expression level does not differ among tissues, may be used as an internal control in the sample. Use of the housekeeping gene mRNA as an internal control has the advantage of being able to detect mRNA of the target gene in a relative manner without regard to the extraction efficiency of the target gene mRNA or the cDNA synthesis efficiency.
Examples of housekeeping gene include the gene for β-actin, a component of the cytoskeleton, and gene for glyceraldehyde-3-phosphate dehydrogenase (hereinafter, referred to as “GAPDH”), a major enzyme in the glycolysis system.
Development of more effective primers for nucleic acid amplification is desired with respect to the housekeeping genes used for the purpose of internal control.
(β-actin Gene)
Action is a protein abundantly found in all the eukaryotic cells. This protein provides a number of structural and regulatory functions, including the role in mitosis, motility and the integrity of the structure of higher eukaryotic cells. Six isoforms of actin have been identified in the vertebrates; four of them are of muscle actin (skeletal muscle, cardiac muscle, aortic smooth muscle and stomach smooth muscle actins) and two of them are of nonmuscle actin (cytoplasmic β- and γ-actin). Muscle actins are tissue-specifically expressed and involved in muscular contraction. In contrast, cytoplasmic actins are found in all the cells in principle and involved in a number of cellular functions. In spite of its diversity, amino acid sequences of the intracellular actin are highly conserved among different actin types and among eukaryotic species.
The sequence of the human cytoplasmic β-actin gene has already been determined and compared with the sequences of β-actin genes derived from other species (Nakajima-Iijima et al, PNAS 82, pp. 6133-6137 (1985); EP Patent Application No. 0174608; Ponte et al., 1984, Nucleic Acids Res. 12, pp. 1687-1696 (1984)). Primers for amplification of the entire human β-actin gene are commercially available from Clontech Laboratories, Inc. (Palo Alto, Calif.) under the name MAPPING Amplimershito for β-actin. Also, with respect to the oligonucleotides that hybridize with the nucleotide sequence of human β-actin in a species-specific manner, it is reported that those oligonucleotides are used as internal controls in the reaction for nucleic acid amplification and tools for determining the integrity of the samples used for nucleic acid amplification (JP, H7-99981-A).
(GAPDH Gene)
Human glyceraldehyde-3-phosphate is one of the important intermediates involved in the glucose metabolism, such as glycolysis and pentose phosphate cycle, and lipogenesis in the living organisms and this substance is widely distributed in the living body. Human glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is also necessary for synthesis of lipids from this substance and this enzyme is also widely distributed in vivo in human.
Development of more effective primers for nucleic acid amplification is desired with respect to β-actin and GAPDH, used for the purpose of internal control.    (Patent Document 1) International Publication: WO 00/28082 (pamphlet)    (Non-Patent Document 1) Bio-Venture, 2001, Vol. 1, No. 1, pp. 109-115    (Non-Patent Document 2) BIO-INDUSTRY, 2001, Vol. 18, No. 2, pp. 15-29